Patent DE-102008005113-A1 (DE)
Title: Protecting the welding quality of spot-welds during the resistance spot welding of predetermined material combination, comprises determining the change of tongs clamping force of welding tongs over welding time for each welding points
Inventor: NIEDERGESAES ULF
IPC: B23K11/24
Filed: 2008-01-15
Abstract: The method comprises determining the change of tongs clamping force (F) of welding tongs over welding time (t s) for each welding points (p) of a material combination (5), where the welding time is related to a reference force (F r e f) given to begin the welding time and the welding points are provided with the welding tongs and classifying the respective spot-weld (p) of the material combination as normal and/or stable, when the determined curve shape of the change of the clamping force has a maximum force (F m a x) before their end and a force (F e n d) at the end of the welding time. The method comprises determining the change of tongs clamping force (F) of welding tongs over welding time (t s) for each welding points (p) of a material combination (5), where the welding time is related to a reference force (F r e f) given to begin the welding time (t s) and the welding points are provided with the welding tongs and classifying the respective spot-weld (p) of the material combination as normal and/or stable, when the determined curve shape of the change of the clamping force has a maximum force (F m a x) before their end and a force (F e n d) at the end of the welding time (t s), where the force (F e n d) is smaller than maximum force (F m a x) but larger than the reference force (F r e f), and when the spot-weld (p) of the material combination is formed with a predetermined strength of the welding current (I s) over the welding time (t s) in a spatter-free manner. The thermal stretch of a system causing the change of the clamping force is overlaid by sinking welding caps (12) into the welding material. After the end of the welding time, the diameter (D l) of the weld point is prognosticated as normal and/or stable classified spot-weld of the given material combination by the condition D l=a+bf m a x+ cf e n d, where: f m a xis relative maximum force (F m a x) related to the reference force (F r e f); f e n dis relative force (F e n d) at the end of the welding time related to the reference force (F r e f); and a, b and c are coefficients. The coefficients (a, b and C) are determined over reference welds of the certain material combination for the welding task by approximating the respective normal and/or stable classified spot-weld, so that the diameter (D l) of the weld point is determined by measurement techniques. The determined diameter (D l) of the weld point is assigned in a three-axis diagram with the coordinates D lmm, f m a xand f e n dto the values for the respective spot-weld under formation of an arrangement of point cloud. A level in the arrangement of point cloud is stretched by the approximation and from that position the coefficients are determined. A readjusting depth delta d of the welding caps and/or the change of the metal sheet force of the given material combination in the respective classified spot-weld and/or the remaining thickness of each spot-weld from the force signal, which is assigned as stable classified spot-weld, are determined according to the prognosis condition delta d=a+bf m a x+cf e n d. The readjusting depth is determined by the measurement of the remaining point thickness of the spot-weld. The determined readjusting depth (delta d) of the weld point is assigned in a three-axis diagram with the coordinates delta d, f m a xand f e n dto the values for the respective spot-weld under formation of an arrangement of point cloud. A transverse cross section represents in the welding plane to determine the diameter (D l) of the weld point and the readjusting depth delta d at each weld point by the measurement of the edges of structure of the weld point and/or the remaining thickness of each spot-weld. The clamping force of the propelled welding tongs is measured by piezosensors (6) arranged at a tong arm (2) of the welding tongs. The welding tongs are servo-motorically, pneumatically or hydraulically propelled.